Anders Behring Breivik has claimed he did not want to carry out the gun massacre that left 69 people dead on the island of Utøya, but was "forced" to do so because Norwegian and EU regulations had made it difficult to acquire sufficient bomb-making equipment.

Giving evidence on the fourth day of his trial, the 33-year-old said he would have preferred to carry out three bomb attacks than target Utøya, where the Norwegian Labour party was holding its annual youth summer camp on 22 July last year. In the end, he went on the rampage on the island after planting one bomb in Oslo's government district, killing eight people.

Attempting to explain his logic, Breivik said: "It's easy to press a button and detonate a bomb. It's very, very difficult to carry out something as barbaric as a firearm-based action."

To do so, he claimed, was difficult. "It is contrary to human nature to execute something like this," he said. "You have to work on yourself for a very long time to make yourself do this … to hammer away at your emotions."

Breivik said he never intended to kill anyone under 18 on Utøya, but that it was hard to assess his victims' ages when so many had their backs to him.

He revealed on Thursday that his original plan for the Utøya attack was to time his arrival on the island with a visit from Gro Harlem Brundtland, a former Labour prime minister of Norway. He told the court he planned to handcuff her before "decapitating" her using a bayonet on his rifle, filming the execution on an iPhone.

"The plan was to chop her head off with [the bayonet] while reading a text and then upload the film to the internet," he said.

Brundtland was his main target, said Breivik, adding that he nonetheless expected everyone on the island to die. "The objective was not to kill 69 people at Utøya. The objective was to kill all of them," he said, adding that he planned to fire shots, scaring the campers into the water where he anticipated they would drown.

"The main goal was to use the water as a mass destruction method. Basically, I assumed most people would drown," he said, adding that it was "hard to swim if you have death anxiety".

Breivik claimed he had chosen decapitation as his preferred method because it is an "old European death penalty method used in Europe".

In the end Brundtland was not on the island when Breivik arrived after detonating the Oslo bomb.

The self-confessed "ultra-nationalist" said he knew he had been criticised for killing minors on Utøya, and that "for 50% of other militant nationalists in Europe Utøya was too much". But those he killed, some as young as 14, were "legitimate targets", he said. Yet he insisted: "I am not a child murderer. I believe that all political activists who choose to fight for multiculturalism … and have leadership positions are legitimate targets."

Breivik was also questioned about why he chose to bomb Oslo's government district, killing eight and seriously injuring many more.

He revealed his original plan was to bomb three targets: Oslo's government district, the Labour party's office and a third target, possibly the royal palace.

"There would be three car bombs, followed by a firearm-based action," said Breivik.

Questioned by prosecutors, Breivik said he had several options for the third target, including the headquarters of the newspapers Aftenposten and Dagsavisen, the Norwegian parliament building and the royal palace.

"I settled on the palace in a setting where the royal family wouldn't be hurt," he said. "Most nationalists and cultural conservatives are supporters of the monarchy, including myself."

The anti-Muslim militant said the three bombs would be followed by several shooting massacres, if he survived. He decided against multiple bombs because building one was "much more difficult than I thought".

Breivik said: "If it hadn't been for the EU and Norway's rules on explosives … there would have been an action that was three car bombs." It was very difficult these days to source the sufficient amount of fertiliser required to make a bomb, he claimed.

He claimed he had "trained" for the "operation" by going on long walks wearing rucksacks filled with 30kg of rocks, aiming to mimic the weight of body armour. He also carried out "reconnaissance missions", scoping out potential targets by walking past them as unobtrusively as possible.